Preparation,modification And Application Of Nitrogen-doped Rice Husk Biochar For Heavy Metal Removal In Organic Wastewater

As a by-product of rice processing,rice husk is usually discarded or incinerated directly,which results in resource waste and air pollution.Rice husk has a high carbon content and is suitable for preparing biochar.In recent years,with the acceleration of industrialization,heavy metal pollution is becoming more and more serious.The preparation of rice husk into biochar can not only sequester most of the carbon in it and reduce carbon dioxide emissions,but also apply the obtained biochar to the treatment of wastewater containing heavy metals.However,the biochar produced by direct pyrolysis of rice husk has a low ability to remove heavy metals and needs to be modified to some extent.In this study,rice husk was used as raw material to prepare biochar.At the same time,nitrogen doped rice husk biochar and iron-supported nitrogen doped rice husk biochar were prepared by modification of urea and ferric chloride during the pyrolysis process.SEM,FTIR,Raman spectroscopy,XRD,XPS and other characterization methods were used to clarify the relationship between pyrolysis temperature and physical and chemical properties of biochar.As（Ⅴ）was selected as the target heavy metal anion,Cu（Ⅱ）as the target heavy metal cation,to explore the removal effect of rice husk biochar on heavy metal ions,the mechanism of action and the removal potential of heavy metals in organic wastewater.The main research contents and results are as follows:（1）Rice husk biochar（NBC-600、NBC-800）was prepared by urea impregnation at 600℃and 800℃.Nitrogen doping enriched the pore structure and surface defects of rice husk biochar,endowed rice husk biochar with higher specific surface area,and reduced the zero charge point of rice husk biochar.With the increase of pyrolysis temperature,the specific surface area of biochar increased and nitrogen content decreased,and the structure nitrogen of biochar increased from three configurations of pyridine nitrogen,pyrrole nitrogen and nitrogen oxide to four configurations of pyridine nitrogen,pyrrole nitrogen,graphite nitrogen and nitrogen oxide.The nitrogen doped rice husk biochar could not significantly improve the removal efficiency of As（Ⅴ）in aqueous solution,but greatly improved the removal efficiency of Cu（Ⅱ）in aqueous solution.When the initial concentration of Cu（Ⅱ）was 100 mg/L and p H=5,the equilibrium adsorption capacity of Cu（Ⅱ）in aqueous solution was the highest with the dosage of 2 g/L biochar,which was 12.7 times of that before modification.The adsorption of Cu（Ⅱ）by nitrogen-doped rice husk biochar was more consistent with the quasi second-order kinetic model and Langmuir isothermal adsorption model,with theoretical maximum adsorption capacities of 66.46 mg/g and 73.82 mg/g,respectively.The action mechanism mainly includes precipitation,electrostatic attraction and physical adsorption.（2）The nitrogen-doped rice husk biochar was impregnated with ferric chloride solution,and the nitrogen-doped rice husk biochar loaded with iron（Fe-NBC-600、Fe-NBC-800）was prepared by pyrolysis method at 600℃and 800℃.Nitrogen is mainly embedded in the carbon skeleton in the form of structural nitrogen,and iron is aggregated or dispersed in the surface and pore of rice husk biochar in the form of iron oxides.The zero charge point of rice husk biochar was reduced by iron loading.Pyrolysis temperature significantly affects the physicochemical properties of nitro-doped rice husk biochar loaded with iron.At 600℃,Fe oxides mainly exist in the form of Fe₂O₃,while at 800℃,Fe oxides mainly exist in the form of Fe₃O₄.The removal rate of As（Ⅴ）from water by iron-loaded nitrogen doped rice husk biochar was up to100%,and the equilibrium adsorption capacity was 10.45 mg/g.The adsorption of As（Ⅴ）by Fe-loaded nitrogen-doped biochar was more consistent with the quasi-second-order kinetic model and Langmuir isothermal adsorption model,and the theoretical maximum adsorption capacity reached 29.64 mg/g.The mechanism of action mainly includes electrostatic attraction,chemical binding of surface-supported iron oxides with As（Ⅴ）,surface complexation of nitrogen-containing functional groups such as structural nitrogen.In the mixed aqueous solution of As（Ⅴ）and Cu（Ⅱ）,the adsorption effect of Fe-supported nitrogen-doped biochar on As（Ⅴ）increased by 1.9times.In organic wastewater containing As（Ⅴ）and Cu（Ⅱ）,the removal effect of rice husk biochar doped with iron-supported nitrogen on As（Ⅴ）and Cu（Ⅱ）was affected by the types of heavy metals and organic matter.The modified rice husk biochar has the potential to be applied to actual organic wastewater.